Do you have the American-spec plugs in your car, or the J-spec plugs?  American-spec spark plugs are just slightly shorter than the J-spec, so they just barely fit on and snap...this could be part of your problem.  As for another possibility -- Engine vibration could be worse at the back of the motor than the front...?  Defective wire that won't lock?  Could be any one or combintation of those I'd say.